1. Field of the Invention
The present invention relates to an antenna duplexer for transmitting data, in which the transmission frequency is the same as the receiving frequency and an antenna is shared.
2. Description of the Related Art
FIG. 7 shows the construction of a conventional antenna duplexer, in which a transmission circuit 31 and a receiving circuit 32 are connected to an antenna 34 via an antenna switching circuit 33. The antenna switching circuit 33 is formed, together with the transmission circuit 31 and the receiving circuit 32, on a printed-circuit board (not shown).
The antenna switching circuit 33 basically has a transmission line 33a having a length of a quarter of the wavelength of the transmission and receiving frequency, and two switching diodes 33b and 33c. The transmission line 33a is formed by a conductor attached to the printed-circuit board. One end of the transmission line 33a and the cathode of the first switching diode 33b are connected to each other, and the connection point thereof is connected to the antenna 34. Also, the anode of the second switching diode 33c is connected to the other end of the transmission line 33a, and the cathode thereof is grounded. The output end of the transmission circuit 31 is connected to the anode of the first switching diode 33b, and the input end of the receiving circuit 32 is connected the other end of the transmission line 33a. 
Then, during transmission, a transmission or receiving switching voltage is applied to the anode of the first switching diode 33b via a choke inductor 33d. Thereupon, the first switching diode 33b and the second switching diode 33c conduct. In this state, a transmission signal output from the transmission circuit 31 passes through the first switching diode 33b and is output to the antenna 34. At this time, since the second switching diode 33c also conducts, the transmission signal input to the transmission line 33a side is totally reflected by the second switching diode 33c and is not input to the receiving circuit 32.
On the other hand, during reception, a switching voltage is not applied to the anode of the first switching diode 33b. Then, the first switching diode 33b and the second switching diode 33c do not conduct. Therefore, the signal received by the antenna 34 is input to the receiving circuit 32 after passing through the transmission line 33a without being input to the transmission circuit 31.
In a conventional antenna duplexer, the antenna switching circuit 33 uses two switching diodes and requires a choke inductor for applying a switching voltage. For this reason, a space for arranging these components must be provided on a printed-circuit board, the printed-circuit board becomes enlarged, and the size of the antenna duplexer cannot be reduced.
Also, since the number of parts used is large, this accordingly results in high cost.
Accordingly, an object of an antenna duplexer of the present invention is to reduce cost by decreasing the number of parts for antenna switching and to reduce the size by reducing the space of a printed-circuit board on which a receiving circuit and a transmission circuit are formed.
To achieve the above-mentioned object, according to the present invention, there is provided an antenna duplexer comprising: an antenna, a transmission circuit, a receiving circuit, a first transmission line for connecting the transmission circuit to the antenna, and a second transmission line for connecting the receiving circuit to the antenna, wherein the length of the first transmission line is set to be approximately a quarter of the wavelength of the transmission frequency, and the length of the second transmission line is set to be approximately a quarter of the wavelength of the receiving frequency.
In the antenna duplexer of the present invention, a first switching diode for shunting a connection point of the first transmission line and the transmission circuit to a ground is provided, and the first switching diode is turned off at the time of transmission and is turned on at the time of reception.
In the antenna duplexer of the present invention, a second switching diode for shunting a connection point of the second transmission line and the receiving circuit to a ground is provided, and the second switching diode is turned off at the time of reception and is turned on at the time of transmission.
In the antenna duplexer of the present invention, a dielectric block in the form of a rectangular parallelepiped is provided, a first strip conductor, and second and third strip conductors which branch from nearly the center along the length of the first strip conductor are formed on the surface of the dielectric block, the first strip conductor is used as the antenna, the second strip conductor is used as the first transmission line, and the third strip conductor is used as the second transmission line.
In the antenna duplexer of the present invention, a first terminal connected to an end portion of the first strip conductor, a second terminal connected to an end portion of the second strip conductor, and a third terminal connected to an end portion of the third strip conductor are formed on the surface of the dielectric block, a printed-circuit board, on which the transmission circuit and the receiving circuit are formed, is provided, the dielectric block is mounted on the printed-circuit board, the first terminal is connected to a grounding conductor provided on the printed-circuit board, the second terminal is connected to the transmission circuit, and the third terminal is connected to the receiving circuit.
The above and further objects, aspects and novel features of the invention will become more fully apparent from the following detailed description when read in conjunction with the accompanying drawings.